Plasma generator

ABSTRACT

An arrangement for generating plasma, the arrangement comprising a primary plasma source ( 1 ) arranged for generating plasma, a hollow guiding body ( 11 ) arranged for guiding at least a portion of the plasma generated by the primary plasma source to a secondary plasma source ( 25 ), and an outlet ( 14 ) for emitting at least a portion of the atomic radicals produced by the plasma from the arrangement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an arrangement and method for removal ofcontaminant deposits, and in particular to a plasma generator forremoving contaminant deposits.

2. Description of the Related Art

The accuracy and reliability of charged particle lithography systems isnegatively influenced by contamination. An important contribution tocontamination in such lithography system is caused by the build-up ofdeposits of contaminants. Charged particle lithography systems generatecharged particles such as electrons, and generate beams of chargedparticles which are focused, modulated and projected onto a wafer in thelithography process. The charged particle beams interact withhydrocarbons present in the lithography system, and the resultingElectron Beam Induced Deposition (EBID) forms a carbon-containing layeron surfaces in the system. This layer of carbon-containing materialaffects the stability of the charged particle beamlets. The chargedparticle beams and beamlets are typically formed using aperture plates,and they may also be focused and modulated by arrays of lenses andelectrodes formed in aperture plates. A build-up of carbon-containinglayers in and around apertures through which the charged particle beamsor beamlets pass also reduces the size of the apertures and reducestransmission of beams or beamlets through these apertures. Removal ofEBID, in particular in areas with relatively high hydrocarbon partialpressures and relatively high beam current densities, is thereforehighly desirable.

Such deposits can be lessened or removed by atomic radical cleaning.This may be achieved using a plasma generator to produce a stream ofatomic radicals that chemically react with the deposits, formingvolatile molecular compounds.

BRIEF SUMMARY OF THE INVENTION

The invention is directed to an improved plasma generator and animproved method for generating plasma. These may be of particularutility in cleaning contaminants such as EBID, and in a charged particlelithography system.

In one aspect the invention provides an arrangement for generatingplasma, the arrangement comprising a primary plasma source arranged forgenerating plasma, a hollow guiding body arranged for guiding at least aportion of the plasma generated by the primary plasma source to asecondary plasma source, and an outlet for emitting at least a portionof the plasma or components thereof (e.g. atomic radicals) from thearrangement. This dual plasma source design enables the plasma generatorto have a larger primary plasma source located remotely from the outletand a smaller secondary plasma source located close to the outlet, dueto the interaction between the two sources. This is particularlyadvantageous in situations where there is limited space at the locationwhere the plasma is required, e.g. for cleaning contaminant deposits onequipment located in a cramped space. The formation of plasma in thesecondary plasma source close to the outlet enables smaller loss ofplasma due to decay of the plasma during transport from the remoteprimary chamber. This design also enables the heat load produced by theprimary plasma source to be located remotely from the outlet.

The primary plasma source may comprise a primary source chamber in whichthe plasma may be formed and a first coil for generating the plasma inthe primary source chamber, the chamber comprising an inlet forreceiving an input gas, and one or more outlets for removal of at leasta portion of the plasma from the source chamber and into the guidingbody. The secondary plasma source may comprise a secondary sourcechamber occupying at least a portion of the guiding body. The secondaryplasma source may omit a coil for enhancing or generating plasma. Thesecondary plasma source may comprise a secondary source chamber and thearrangement may be adapted to generate a high brightness plasma in thesecondary source chamber.

A plasma generator may operate by capacitive coupling where an electricfield is generated by a radio frequency (RF) voltage between twoelectrodes which induces the plasma formation, or by inductive couplingwhere a magnetic field is generated by an RF current through a coilwhich induces the plasma formation. In some embodiments, in operation,the primary plasma source may be adapted to generate a primary plasmavia inductive coupling, and the secondary plasma source to generate asecondary plasma via capacitive coupling. The arrangement thus forms ahybrid plasma generator using both inductive and capacitive coupling togenerate plasma. The arrangement may further comprise an electrodelocated near the outlet of the arrangement, wherein, in operation, thecoil of the primary plasma source is capacitively coupled to theelectrode via the plasma generated by the primary plasma source and/orthe secondary plasma source. The electrode may be maintained at a fixedpotential with respect to a voltage supplied to the coil of the primaryplasma source, or it may be grounded with respect to a voltage suppliedto the coil of the primary plasma source.

The arrangement may also comprise an aperture array near the outlet, andmay also comprise an additional electrode arranged for repelling orattracting plasma ions in the guiding body. The plasma formed by theplasma generator includes ions and radicals, and this arrangementenables retention of ions in the plasma generator or reduction of theirenergy, while permitting emission of radicals from the plasma generator.

The primary plasma source may comprise a primary source chamber in whichprimary plasma is generated and the secondary plasma source may comprisea secondary source chamber in which the primary plasma is enhancedand/or secondary plasma is generated, the primary source chamber beinglarger than the secondary source chamber. The primary source chamber mayhave a larger cross-section than the secondary source chamber, and mayhave a greater internal volume. The larger primary source chamber canthen be located further from the outlet than the secondary sourcechamber, allowing constructions where the smaller secondary sourcechamber can fit into narrow restricted spaces close to the locationwhere the plasma is required.

The arrangement may further comprise a pressure regulator for regulatingpressure in the primary source chamber, and a flow or pressurerestriction may be provided between the primary and secondary sourcechambers. The restriction may be adapted to maintain an operatingpressure in the secondary source chamber at a lower pressure than in theprimary source chamber. The arrangement may also be adapted forregulating the pressure in the secondary source chamber, or forregulating the pressure in both the primary and secondary sourcechambers.

The secondary source chamber may have a length longer than the primarysource chamber in a direction of the flow of plasma from the primarysource chamber. The primary source chamber may have a diameter of 20 mmor more, and the secondary source chamber may have a diameter of lessthan 20 mm. The secondary source chamber may have an end section fordirecting plasma in a desired direction.

The secondary source chamber may be arranged to generate plasma at aposition close to an outlet of the arrangement, and the primary plasmasource may be located further from the outlet than the secondary plasmasource. This results in a design with a secondary plasma chamber closerto the outlet where the plasma is emitted, so that less of the plasmagenerated in the secondary chamber is lost by decay and other processesduring transfer to the outlet.

The hollow guiding body may comprise a funnel section located at theoutlet of the primary plasma source arranged for guiding plasmagenerated by primary plasma source into the guiding body. The guidingbody may comprise a quartz material or an inner surface comprising aquartz material, and the guiding body may be in the form of a tube orduct. The guiding body may have a bend or elbow to direct plasma fromthe outlet onto an area to be cleaned by the plasma.

The primary plasma source may comprise a primary source chamber in whichthe plasma may be formed, and an aperture plate positioned between theprimary source chamber and the guiding body, the aperture plate havingone or more apertures for permitting flow of the plasma from the primarysource chamber into the guiding body. The arrangement may furthercomprise an aperture plate at or near the outlet of the guiding body toconfine at least a portion of the plasma in the guiding body fromexiting through the outlet.

In another aspect the invention relates to a method for generating aplasma, comprising flowing an input gas into a primary source chamber,energizing a first coil to form a primary plasma in the primary sourcechamber, flowing at least a portion of the primary plasma into asecondary source chamber, and generating a secondary plasma in thesecondary source chamber. The step of flowing the primary plasma intothe secondary source chamber may comprise flowing the plasma into aguiding body, at least a portion of the guiding body forming thesecondary source chamber. The first plasma may be flowed from theprimary source chamber through a restriction into a secondary sourcechamber.

The method may comprise forming the primary plasma in the primary sourcechamber via inductive coupling, and generating the secondary plasma inthe secondary source chamber via capacitive coupling.

The secondary source chamber may omit a coil for forming a plasma. Themethod may further comprise regulating pressure in the primary sourcechamber and the secondary source chamber, and the step of regulating thepressure may comprise maintaining a lower pressure in the secondarysource chamber than in the primary source chamber. The primary plasmamay be a relatively low brightness plasma and the secondary plasma maybe a relatively high brightness plasma.

The method may further comprise stabilizing the formation of plasma inthe secondary source chamber with the primary plasma flowing from theprimary source chamber, and may further comprise maintaining a lowerpressure in the secondary source chamber than in the primary sourcechamber.

In another aspect the invention relates to a cleaning apparatus forcleaning contaminants from a surface, the apparatus comprising anarrangement for generating plasma as described herein, and means fordirecting the plasma onto the surface to be cleaned.

In yet another aspect the invention relates to a charged particlelithography machine, comprising a beamlet generator for generating aplurality of charged particle beamlets and a plurality of beamletmanipulator elements for manipulating the beamlets, each beamletmanipulator element comprising a plurality of apertures through whichthe beamlets pass, the machine further comprising an arrangement forgenerating plasma as described herein, adapted to generate plasma anddirect the plasma onto a surface of one or more of the beamletmanipulator elements.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in which:

FIG. 1 is a schematic diagram of an embodiment of a radio frequency (RF)plasma generator;

FIGS. 2A, 2B and 2C are schematic diagrams of an embodiment of a plasmagenerator including a guiding body;

FIGS. 3A and 3B are schematic diagrams of the embodiment of FIG. 2 inoperation;

FIG. 4 is a schematic diagram of an embodiment including aperture platesand an electrode at the outlet;

FIG. 5 is a schematic diagram of another embodiment including apertureplates and electrodes at the outlet;

FIG. 6 is a photograph of a plasma chamber with a guiding body showingplasma forming in the primary plasma chamber;

FIG. 7 is a photograph of the plasma chamber of FIG. 6 showing plasmaforming in the guiding body; and

FIG. 8 is a schematic diagram of an embodiment of a charged particlelithography machine.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The following describes certain embodiments of the invention, given byway of example only and with reference to the figures.

FIG. 1 shows a radio frequency (RF) plasma generator comprising achamber 2 with an RF coil 4 around the outside of the chamber. An inputgas such as oxygen or hydrogen or other suitable gas is passed into thechamber via inlet 5 and the coil 4 is energized with an RF voltage toproduce a plasma including radicals, such as oxygen atom radicals, whichexit the chamber via one or more outlets 6. In the followingdescription, except where the context indicates otherwise, the termplasma is used for simplicity to denote a plasma and/or radicalsproduced in such a plasma generator.

Contaminants such as electron beam induced deposition (EBID), generallycomprising carbon containing compounds, form on surfaces in a chargedparticle lithography system, such as the surfaces of beamlet manipulatorelements (such as beamlet modulators, deflectors, lenses, aperturearrays, beam stop arrays, etc.). For removing contaminants such as EBIDdeposits, radicals, such as oxygen atom radicals, may be used, reactingwith carbon in the EBID deposits to form carbon monoxide. Plasmatypically comprises a mixture of gas molecules, ions, electrons, andatomic radicals. For cleaning EBID deposits, atomic ions may also beused. However, due to their electrical charge, ions may be acceleratedby electric fields generated in or around the plasma generator system,and have sufficient kinetic energy to sputter the surface to be cleaned.This can result in removing not just the contaminant deposits, but alsopart of the surface underlying the deposits, so damaging the surface.Uncharged radicals generally have a lower kinetic energy. i.e. thethermal energy of the radicals, and are more suitable in many cleaningapplications for this reason.

Excellent cleaning rates of greater than 5 micron per hour can beachieved by atomic radical cleaning where there is a direct view fromthe plasma generator source to the contaminated area to be cleaned.However, this method cannot be easily implemented in situations wherethe deposits are formed on surfaces with cannot be easily accessed andin which there is little room to locate the plasma generator with adirect line from the source to the area to be cleaned. The beam stoparray and beamlet aperture array are areas that exhibit these problems,typically having a restricted space available above the surface of thebeam stop of 5 mm of less. Typically, plasma sources are constructedwith a long tube of length 20 cm or more and a diameter of about 10 cm.

For example, for cleaning the beam stop array and beamlet aperture arrayin a charged particle lithography system there is typically very limitedspace (e.g. about 10×10×10 mm³) available to implement a plasma sourcein the vicinity of these elements. The problem in designing a miniatureplasma source for use in locations with very restricted space lies inthe aspect ratio of the area to volume of the plasma generator. For alarge plasma source this ratio is small, but as the size of the plasmasource is reduced the ratio increases. This results in minorinstabilities coupling into or out of the plasma in the source chambervia its surface, having an increasingly large effect. As a consequence,the plasma may be hard to ignite and easily extinguished due to theseinstabilities.

Instead, a much larger source may be used (e.g. about 100×100×100 mm³)to be situated at approximately 200 mm from the elements to be cleaned.The plasma radicals must then be transported from the plasma generatorto the site of the cleaning.

The present invention provides a plasma generator which permits directaccess to the contaminated area even where only a limited volume isavailable at the cleaning site to locate the cleaning apparatus. Aplasma source is placed near the contaminated area to be cleaned, and aguiding path is attached to the plasma generator and the plasma radicalsproduced are transported towards the area to be cleaned.

FIGS. 2A, 2B and 2C schematically show arrangements for removal ofcontaminant deposits, in particular for removal of contaminantsdeposited on surfaces that are located in areas with restricted ordifficult access. The arrangements comprise a plasma generator similarto that shown in FIG. 1, further referred to as the primary sourcechamber 15, which functions as a primary plasma source 1. Thearrangement further comprises a hollow guiding body 11, such as tube orduct, for guiding plasma towards a predetermined destination area. Itwill be recognized that various configurations are possible, and threepossible configurations are shown, and features of any of theseembodiments may be used in any of the other embodiments. The arrangementin FIG. 2A the guiding body 11 includes a funnel portion 10 where theguiding body 11 is coupled to the primary source chamber 15, and thechamber end wall with outlets 6 function as an aperture plate to permitthe plasma and radicals to enter the guiding body 11 from the chamber15. The guiding body 11 includes an elbow 12 near the end of the guidingbody to direct plasma exiting the guiding body. The arrangement in FIG.2B omits the funnel portion, the chamber end wall, and the elbow, sothat a straight guiding body 11 having a smaller cross section than theprimary source chamber 15 is coupled directly to the primary sourcechamber 15. The arrangement in FIG. 2C features a continuous hollowbody, a wider portion forming the primary source chamber 15 and anarrower portion forming the guiding body 11. A hollow body having auniform cross section may also be used, one portion of the hollow bodyfunctioning as the primary source chamber 15 and another portionfunctioning as the guiding body 11.

The guiding body 11 may be straight or may comprise one or more bendssuch as an elbow 12 or bend 13 to direct the plasma in a desireddirection. Preferably, the guiding body 11 is as straight as possible toincrease the average lifetime of radicals being transferred through thetube. The guiding body has an outlet 14 which may be located in closeproximity of the contaminant deposit to be reduced or removed.Typically, the outlet 14 is in direct contact with a vacuum environment.

The plasma and radicals generated in primary source chamber 15 ofprimary source 1 are guided towards the contaminant deposit to bereduced or removed via the guiding body 11. The guiding body 11 may bemade of quartz, or with inner surface coated with quartz, to suppressextinction of the radicals when they interact with these parts of thedevice. Embodiments of the invention are described herein with referenceto plasma formed from oxygen. However, it will be understood that theinvention may also employ plasmas from other gases, such as hydrogen ornitrogen.

FIG. 3A shows the arrangement of FIG. 2A in operation. It will beunderstood that arrangements shown in FIGS. 2B and 2C may also beoperated in a similar manner. Oxygen is supplied to the primary sourcechamber 15 and RF coil 4 is energized to inductively heat the oxygen,and a plasma 20 is generated in primary source chamber 15. The oxygenpressure may be adjusted, for example, to produce a relatively highpressure in the chamber 15. The plasma 20, and in particular radicalsproduced therein, may exit the primary source chamber 15 asschematically represented by dashed arrow 21 and flow into the guidingbody 11.

Major losses of radicals are typically observed in these arrangementsduring transport from the primary source chamber 15 to the outlet 14 atthe site where the cleaning is to take place. Several processes willcause annihilation of the atomic radicals, such as volume recombination,surface adsorption and surface recombination. The losses of such asystem are significant, e.g. using power of 600 W for the sourcechamber, the efficiency of transport of the radicals is only 0.4%. Thelosses in the guiding body 11 can be compensated by using a more intenseplasma source, but the thermal load caused by using such high power forthe plasma generator becomes a serious problem for many applications,particularly when used in a vacuum environment as required forlithography applications. By carefully designing the pressure andtemperature of the primary source chamber and guiding body, the lossescan be minimized.

The plasma generator may also be made more effective by more efficienttransport of plasma through the guiding body 11, and/or generation ofplasma in the guiding body 11 so that the guiding body is not merelyconveying plasma formed in the primary source chamber 15, but additionalplasma is formed in the guiding body, close to the location where it isneeded for cleaning. In this case a secondary plasma source 25 isformed, with a portion of the guiding body functioning as a secondarysource chamber 16.

This may be accomplished in different ways. By adjusting pressure in theprimary source chamber and pressure in the guiding body relative to thesurrounding environment, plasma may be guided from the primary sourcechamber 15 into the guiding body 11 and through the guiding body to theoutlet 14. The pressure should preferably decrease from the primarysource chamber 15 to the guiding body 11 and to the environment outsidethe outlet 14 in order to promote flow of plasma from the chamber 15 tothe outlet 14. Depending on the plasma formation, the pressure can beoptimized through the device, e.g. by use of aperture plates (such asaperture plate 31 at the entrance to the guiding body 11 and apertureplate 32 at the outlet 14) and/or by adjusting the relative sizes andgeometry of the primary source chamber 15, guiding body 11 and theoutlet 14.

By adjusting pressures in the primary source chamber 15 and in theguiding body 11, plasma can be formed in the guiding body. The pressuresmay be adjusted to obtain conditions where a high brightness plasma isformed in the guiding body at a relatively lower pressure than theprimary source chamber. This results in the formation of plasma in theguiding body closer to the location where it is needed to produce moreeffective cleaning.

The pressure in the guiding body 11 may be relatively low in comparisonto the primary source chamber 15, depending on the geometry of the tubeand size of the outlet 14, and the ambient pressure of the surroundingenvironment. A relatively high pressure in the primary source chamber 15will displace more plasma into the guiding body 11, thus shortening thepath for the radicals to reach the outlet 14 and the cleaning site, andthus reducing the loss of radicals due to recombination and othereffects, and thereby increasing the cleaning rate.

In a lithography machine cleaning application, the ambient pressure maybe low inside the lithography machine vacuum chamber, e.g. 10⁻³ millibaror lower. The pressure in the guiding body 11 may be higher, e.g. 10⁻²millibar, but lower than the pressure in the primary plasma chamber 15,aiding the formation of plasma in the guiding body itself, even thoughthere is no RF coil around the guiding body. This may be due to therelatively lower pressure in the guiding body 11 and may be assisted bythe flow of plasma from the primary source chamber 15, conveying theeffect of the RF coil at the primary source chamber, e.g. by capacitivecoupling, into the guiding body. Plasma is an electrical conductor, andthus may conduct RF current resulting from excitation of the RF coils 4surrounding the primary source chamber 15, into the guiding body 11where it may generate more plasma.

The resulting effect is to generate plasma in the guiding body 11, aportion of the guiding body functioning as a “passive” secondary plasmasource 25. The guiding body is not merely conveying plasma formed in byprimary plasma source 1, but additional plasma is formed in a secondaryplasma source 25 in the guiding body 11, close to the location where itis needed for cleaning. By adjusting the relative pressures in thesystem, tuned by adjusting the entry pressure of the input gas to theprimary source chamber 15, and adapting the dimensions of the guidingbody 11 and outlet 14, the plasma formed in the secondary plasma source25 may be high brightness plasma suitable for effective cleaning of theEBID deposits. FIG. 4 shows this effect in which plasma 20 is formed inthe primary source chamber 15, plasma (or components of the plasma) 21flows into the guiding body 11, and plasma 22 is subsequently formed ina secondary source chamber 16 in the guiding body 11. The plasma 21 actsas “seed” plasma to enhance plasma formation in the secondary sourcechamber 16 near to the outlet 14, where it acts as a source of atomicradicals close to the location where they are needed.

An alternative is to operate the system at a low pressure in the primarysource chamber 15 and guiding body 11 to reduce the loss of radicals bydecreasing the recombination probability, and thereby increasing thecleaning rate.

The embodiment shown in FIG. 4 employs an optional aperture plate 31between the primary source chamber 15 and guiding body 11 at the exit ofthe primary source. The gas flow conductance of the aperture plate 31can be adjusted (by adjusting the number and size of the apertures inthe plate) to adjust the relative pressures in the source chamber 15 andguiding body 11. The aperture plate 31 also operates to partly confinethe plasma in the primary source chamber 15 by reducing the quantity ofions flowing from the chamber 15 into the guiding body 11, due partly torecombination of ions and electrons due to collisions, while permittingflow of radicals into the guiding body 11. The aperture plate 31 may beomitted altogether where maximum flow of plasma into guiding body 11 isdesired to maximize formation of secondary plasma 22 in the guiding body11.

An optional aperture plate 32 may also be placed near the end of theguiding body 11 (as shown in the FIG. 5 embodiment), preferably at theoutlet 14. The aperture plate 32 at the outlet 14 can be used to assistin regulating pressure in the guiding body 11, typically in conjunctionwith aperture plate 31. Aperture plate 32 may also operate to partlyconfine the plasma inside the guiding body 11, limiting the ability ofhigh kinetic energy ions from striking the area to be cleaned, or thisfunction may be performed by the electrode 32. These high energy ionsmay sputter the top surface of the area to be cleaned and this can leadto damage of the part being cleaned. Furthermore, when both apertureplates 31 and 32 are used, the gas flow conductance of the apertureplates can be adjusted so that the pressure in both the primary sourcechamber 15 and the guiding body 11 (i.e. secondary source chamber 16) isoptimal, to improve the efficiency of the plasma generator.

The aperture plates 31 and/or 32 may be made from a conducting materialsuch as a metal, or a non-conducting material such as a plastic, ceramicor quartz. The aperture plate 32 may be made from a conducting materialto function also as an electrode, which may be grounded or connected toa common voltage. Alternatively, a separate electrode 30 may beinstalled near the outlet 14, as shown in the embodiment of FIG. 4,which may be grounded or connected to a common voltage.

Such an electrode 30 (or grounded aperture plate 32) near the outlet 14operates to generate plasma in the guiding body 11 and/or enhance plasmaflowing into the guiding body 11, through capacitive coupling. Plasma isa conductor, and as it flows from the primary source chamber 15 into theguiding body 11 towards the electrode 30, it creates a capacitivecoupling between the RF coil 4 of the primary plasma source 1 and theelectrode 30 at the outlet 14 at the end of the secondary source chamber25. The RF voltage supplied to the RF coil 4 generates an electric fieldbetween the RF coil 4 and electrode 30 conducted by the seed plasma 21flowing into the guiding body 11, which excites the plasma in theguiding body 11 between the RF coil 4 and electrode 30 which enhances orinduces formation of plasma 22 in the guiding body 11. Plasma generationvia such capacitive coupling is typically difficult to achieve unlessthe guiding body is short, i.e. short distance from the primary plasmasource to the cleaning site, particularly when there is grounded metalnear to the plasma generator, which is usually the case where the plasmagenerator is surrounded by other equipment. The electrode 30 may alsooperate to partly confine the plasma 22 inside the guiding body 11, e.g.taking the form of a mesh or aperture plate. The electrode 40 alsofunctions to avoid or reduce capacitive coupling between the RF coil 4and the part being cleaned where the part is conductive and grounded.This can avoid damage to the part being cleaned where it is vulnerableto stray electrical current, e.g. a beamlet modulation array of acharged particle lithography machine.

However, a “hybrid” plasma generator, which uses both inductive andcapacitive coupling to generate plasma as described herein, can overcomethis difficulty. The system generates a primary plasma 20 in the primarysource chamber 15 using inductive coupling, where a magnetic fieldgenerated by an RF current through the coil 4 induces plasma formation,and a secondary plasma 22 in the guiding body 11/second plasma chamber16 using capacitive coupling, where an electric field is generated by anRF voltage between the coil 4 and electrode 30 induces plasma formation.The primary inductively-coupled plasma can “grow” in the guiding body 11towards the electrode 30 near the cleaning site, changing frominductively coupled to capacitively coupled. This process starts with aprimary inductively-coupled plasma that can be formed in a groundedenvironment, i.e. where there are grounded conductors nearby. In such agrounded environment, it is very difficult to sustain acapacitively-coupled plasma. The primary plasma 20 in the primary sourcechamber 15 heats the nearby volume in the guiding body 11, partly due toflow of hot plasma 21 into the guiding body, and the plasma grows alittle more in the guiding body. The plasma is a conductor and itsgrowth/formation in the guiding body extends the electric field from theRF coil 4 of the primary source 1 further into the guiding body 11,aiding further plasma growth/formation. This process continues until theplasma reaches the electrode 30 and a high brightness plasma can beformed in the guiding body 11/second plasma chamber 16.

The embodiment in FIG. 4 has a bend 13 at the end of the guiding body 11near the outlet 14, rather than a 90 degree elbow 12, while theembodiment of FIG. 5 has a straight guiding body 11. Any of theconfigurations shown in any of the drawings may be used with any of theembodiments, with or without the funnel section 10 or aperture plates 31and/or 32 or electrodes 30 and/or 34.

The embodiment shown in FIG. 5 employs an additional electrode 34 tofurther reduce the quantity of atomic ions exiting the outlet 14 of theplasma generator and/or reduce their velocity. The additional electrode34 may be energized with a voltage V to repel or attract the ions, e.g.a positive voltage to repel negative ions, or an RF voltage opposite tothe voltage supplied to the RF coil 4. Note that the electrode 30,aperture plate 32, and additional electrode 34 may be used in variouscombinations to achieve the desired results, i.e. to extend plasmaformation into the guiding body 11 while controlling the emission ofenergetic ions while permitting the emission of atomic radicals. Theseelements operate as a atomic radical/ion filter, to let radicals passwhile reducing or preventing ion emission.

This new design provides an arrangement with two plasma sources, asmaller secondary plasma source 25 located close to the plasma outlet 14and a larger primary plasma source 1 further from the outlet 14, thearrangement including a guiding body 11 for guiding plasma generated bythe primary plasma source 1 to the secondary plasma source 25 tostabilize plasma formation by the secondary plasma source 25. The designis simple with very few parts, the secondary plasma source 25 operatingby capacitive coupling to an electrode 30/aperture plate 32.

FIG. 6 is a photograph of a plasma chamber in the center of thephotograph, with an guiding body in the form of an extension tubeextending towards the bottom of the photograph. Greenish low brightnessplasma is flowing from the primary plasma chamber into the guiding bodytowards the outlet at the bottom of the photograph.

FIG. 7 is a photograph of the plasma chamber of FIG. 6 at the top of thephotograph, with the guiding body extending towards the bottom of thephotograph. The faint greenish low brightness plasma in the guiding bodyhas been replaced by high brightness plasma which is forming in theguiding budy and exiting the outlet at the bottom of the photograph.

FIG. 8 shows a simplified schematic diagram of an electron-opticalcolumn of a charged particle lithography element. Such lithographysystems are described for example in U.S. Pat. Nos. 6,897,458;6,958,804; 7,019,908; 7,084,414; and 7,129,502, U.S. patent publicationno. 2007/0064213, and co-pending U.S. patent application nos.61/031,573; 61/031,594; 61/045,243; 61/055,839; 61/058,596; and61/101,682, which are all assigned to the owner of the present inventionand are all hereby incorporated by reference in their entireties.

In the embodiment shown in FIG. 8, the lithography element columncomprises an electron source 110 producing an expanding electron beam130, which is collimated by collimator lens system 113. The collimatedelectron beam impinges on an aperture array 114 a, which blocks part ofthe beam to create a plurality of sub-beams 134, which pass through acondenser lens array 116 which focuses the sub-beams. The sub-beamsimpinge on a second aperture array 114 b which creates a plurality ofbeamlets 133 from each sub-beam 134. The system generates a very largenumber of beamlets 133, preferably about 10,000 to 1,000,000 beamlets.

A beamlet blanker array 117, comprising a plurality of blankingelectrodes, deflects selected ones of the beamlets. The undeflectedbeamlets arrive at beam stop array 118 and pass through a correspondingaperture, while the deflected beamlets miss the corresponding apertureand are stopped by the beam stop array. Thus, the beamlet blaker array117 and beam stop 118 operate together to switch the individual beamletson and off. The undeflected beamlets pass through the beam stop array119, and through a beam deflector array 119 which deflects the beamletsto scan the beamlets across the surface of target or substrate 121.Next, the beamlets pass through projection lens arrays 120 and areprojected onto substrate 121 which is positioned on a moveable stage forcarrying the substrate. For lithography applications, the substrateusually comprises a wafer provided with a charged-particle sensitivelayer or resist layer.

The lithography element column operates in a vacuum environment. Avacuum is desired to remove particles which may be ionized by thecharged particle beams and become attracted to the source, maydissociate and be deposited onto the machine components, and maydisperse the charged particle beams. A vacuum of at least 10⁻⁶ bar istypically required. In order to maintain the vacuum environment, thecharged particle lithography system is located in a vacuum chamber. Allof the major elements of the lithography element are preferably housedin a common vacuum chamber, including the charged particle source,projector system for projecting the beamlets onto the substrate, and themoveable stage.

The invention has been described by reference to certain embodimentsdiscussed above. It will be recognized that these embodiments aresusceptible to various modifications and alternative forms well known tothose of skill in the art without departing from the spirit and scope ofthe invention. Accordingly, although specific embodiments have beendescribed, these are examples only and are not limiting upon the scopeof the invention, which is defined in the accompanying claims.

What is claimed is:
 1. An arrangement for generating plasma, thearrangement comprising a primary plasma source (1) arranged forgenerating plasma, a hollow guiding body (11) arranged for guiding atleast a portion of the plasma generated by the primary plasma source toa secondary plasma source (25), and an outlet (14) for emitting at leasta portion of the plasma and components thereof from the arrangement. 2.The arrangement of claim 1, wherein the primary plasma source (1)comprises a primary source chamber (15) in which the plasma may beformed and a first coil (4) for generating the plasma in the primarysource chamber, the chamber comprising an inlet (5) for receiving aninput gas, and one or more outlets for removal of at least a portion ofthe plasma from the source chamber and into the guiding body (11). 3.The arrangement of claim 1 or 2, wherein the secondary plasma source(25) comprises a secondary source chamber (16) occupying at least aportion of the guiding body (11).
 4. The arrangement of any one of thepreceding claims, wherein the secondary plasma source (25) does notinclude a coil for enhancing or generating plasma.
 5. The arrangement ofany one of the preceding claims, wherein the secondary plasma source(25) comprises a secondary source chamber (16) and wherein thearrangement is adapted to generate a high brightness plasma in thesecondary source chamber.
 6. The arrangement of any one of the precedingclaims, wherein, in operation, the primary plasma source (1) generates aprimary plasma via inductive coupling, and the secondary plasma source(25) generates a secondary plasma via capacitive coupling.
 7. Thearrangement of any one of the preceding claims, further comprising anelectrode (30, 32, 34) located near the outlet of the arrangement,wherein, in operation, the coil (4) of the primary plasma source (1) iscapacitively coupled to the electrode via the plasma generated by theprimary plasma source and/or the secondary plasma source.
 8. Thearrangement of claim 7, wherein the electrode is maintained at a fixedpotential with respect to a voltage supplied to the coil (4) of theprimary plasma source (1).
 9. The arrangement of claim 7, wherein theelectrode is grounded with respect to a voltage supplied to the coil (4)of the primary plasma source (1).
 10. The arrangement of claim 9,further comprising an additional electrode (34) arranged for repellingplasma ions in the guiding body (11).
 11. The arrangement of any one ofthe preceding claims, wherein the primary plasma source (1) comprises aprimary source chamber (15) in which primary plasma is generated and thesecondary plasma source (25) comprises a secondary source chamber (16)in which the primary plasma is enhanced and/or secondary plasma isgenerated, and wherein the primary source chamber is larger than thesecondary source chamber.
 12. The arrangement of claim 11, furthercomprising a pressure regulator for regulating pressure in the primarysource chamber, and wherein a flow or pressure restriction is providedbetween the primary and secondary source chambers.
 13. The arrangementof claim 12, wherein, in use, the restriction is adapted to maintain anoperating pressure in the secondary source chamber at a lower pressurethan in the primary source chamber.
 14. The arrangement of any one ofclaims 11-13, wherein the arrangement is adapted for regulating thepressure in the secondary source chamber.
 15. The arrangement of any oneof claims 11-14, wherein the arrangement is adapted for regulating thepressure in both the primary and secondary source chambers.
 16. Thearrangement of any one of claims 11-15, wherein the primary sourcechamber has a diameter of 20 mm or more, and the secondary sourcechamber has a diameter of less than 20 mm.
 17. The arrangement of anyone of claims 11-16, wherein the secondary source chamber has an endsection for directing plasma in a desired direction.
 18. The arrangementof any one of claims 11-17, wherein the secondary source chamber has arelatively small cross-sectional area in comparison with the primarysource chamber.
 19. The arrangement of any one of claims 11-18, whereinthe secondary source chamber is a tube.
 20. The arrangement of any oneof claims 11-19, wherein the secondary source chamber has a lengthlonger than the primary source chamber in a direction of the flow ofplasma from the primary source chamber.
 21. The arrangement of any oneof claims 11-20, wherein the secondary source chamber generates plasmaat a position close to an outlet of the arrangement.
 22. The arrangementof any one of the preceding claims, wherein the primary plasma source(1) is located further from the outlet (14) than the secondary plasmasource (25).
 23. The arrangement of any one of the preceding claims,wherein, in operation, at least a portion of the plasma generated by theprimary plasma source (1) travels through the guiding body (11) to thesecondary plasma source (25) to stabilize the formation of plasma by thesecondary plasma source (25).
 24. The arrangement of any one of thepreceding claims, wherein the hollow guiding body (11) comprises afunnel section (10) located at the outlet of the primary plasma source(1) arranged for guiding plasma generated by primary plasma source intothe guiding body.
 25. The arrangement of any one of the precedingclaims, wherein the guiding body (11) comprises a quartz material or hasan inner surface comprising a quartz material.
 26. The arrangement ofany one of the preceding claims, wherein the guiding body (11) is in theform of a tube or duct.
 27. The arrangement of any one of the precedingclaims, wherein the guiding body (11) has a bend (13) or elbow (12) todirect plasma from the outlet (14) onto an area to be cleaned by theplasma.
 28. The arrangement of any one of the preceding claims, whereinthe primary plasma source (1) comprises a primary source chamber (15) inwhich the plasma may be formed, the arrangement further comprising anaperture plate (31) positioned between the primary source chamber andthe guiding body (11) and the aperture plate having one or moreapertures for permitting flow of the plasma from the primary sourcechamber into the guiding body.
 29. The arrangement of any one of thepreceding claims, further comprising an aperture plate (32) at or nearthe outlet (14) of the guiding body to confine at least a portion of theplasma in the guiding body from exiting through the outlet.
 30. A methodfor generating a plasma, comprising flowing an input gas into a primarysource chamber, energizing a first coil to form a primary plasma in theprimary source chamber, flowing at least a portion of the primary plasmainto a secondary source chamber, and generating a secondary plasma inthe secondary source chamber.
 31. The method of claim 30, whereinflowing the primary plasma into the secondary source chamber comprisesflowing the plasma into a guiding body, at least a portion of theguiding body forming the secondary source chamber.
 32. The method ofclaim 30 or 31, wherein the first plasma is flowed from the primarysource chamber through a restriction into a secondary source chamber.33. The method of any one of claims 30-32, wherein the secondary sourcechamber is not provided with a coil for forming a plasma.
 34. The methodof any one of claims 30-33, wherein the primary plasma is formed in theprimary source chamber via inductive coupling, and the secondary plasmais generated in the secondary source chamber via capacitive coupling.35. The method of any one of claims 30-34, further comprising regulatingpressure in the primary source chamber and the secondary source chamber.36. The method of claim 35, wherein regulating the pressure comprisesmaintaining a lower pressure in the secondary source chamber than in theprimary source chamber.
 37. The method of any one of claims 30-36,wherein the secondary source chamber is smaller than the primary sourcechamber.
 38. The method of any one of claims 30-37, further comprisingstabilizing the formation of plasma in the secondary source chamber withthe primary plasma flowing from the primary source chamber.
 39. Ancleaning apparatus for cleaning contaminants from a surface, theapparatus comprising an arrangement for generating plasma according toany one of claims 1-29, and means for directing the plasma onto thesurface to be cleaned.
 40. A charged particle lithography machine,comprising a beamlet generator for generating a plurality of chargedparticle beamlets and a plurality of beamlet manipulator elements formanipulating the beamlets, each beamlet manipulator element comprising aplurality of apertures through which the beamlets pass, the machinefurther comprising an arrangement according to any one of claims 1-29adapted to generate plasma and direct the plasma onto a surface of oneor more of the beamlet manipulator elements.